Greenland was once actually green - even if it was close to a million years ago. But its lack of an ice sheet back then may lead to clues about the increased risk of sea-level rise in a warmer future.
A new study provides the first direct evidence that the center, not just the edges, of Greenland's ice sheet melted away in the recent geological past and the now-ice-covered island was then home to a green, tundra landscape.
A team of scientists, including Lawrence Livermore National Laboratory (LLNL) researchers, examined a few inches of sediment from the bottom of a two-mile-deep ice core extracted at the very center of Greenland in 1993 and held for 30 years in a Colorado storage facility. They were amazed to discover soil that contained willow wood, insect parts, fungi and a poppy seed in pristine condition.
"These fossils are clear evidence of ice-free times in Greenland," said LLNL scientist and co-author Alan Hidy. Hidy and his colleague Greg Balco analyzed isotopes in the samples to determine their age using LLNL's Center of Accelerator Mass Spectrometry.
"The glacial till showed that the Greenland Ice Sheet at the summit is long-lived, stable land surface preserving a record of deposition, exposure and interglacial ecosystems."
The study, published in the Proceedings of the National Academy of Sciences, confirms that Greenland's ice melted, and the island greened during a prior warm period likely within the last million years, suggesting that the giant ice sheet is more fragile than scientists had realized until the last few years.
Sea level today is rising more than an inch each decade. "And it's getting faster and faster," said Paul Bierman, a professor at the University of Vermont who co-led the study. It is likely to be several feet higher by the end of this century, when today's children are grandparents. And if the release of greenhouse gases, from burning fossil fuels, is not radically reduced, he said, the near complete melting of Greenland's ice over the next centuries to a few millennia would lead to some 23 feet of sea level rise.
Core assumptions
In 2016, researchers at Columbia University tested rock from the bottom of the same 1993 ice core (called GISP2) and published a then-controversial study suggesting that the current Greenland ice sheet could be no more than 1.1 million years old; that there were extended ice-free periods during the Pleistocene (the geological period that began 2.7 million years ago); and that if the ice was melted at the GISP2 site, then 90% of the rest of Greenland would be melted also. This was a major step toward overturning the longstanding story that Greenland is an implacable fortress of ice, frozen solid for millions of years.
Then, in 2019, Bierman and an international team, including Lawrence Livermore researchers, reexamined another ice core, this one extracted at Camp Century near the coast of Greenland in the 1960s. They were stunned to discover twigs, seeds and insect parts at the bottom of that core, revealing that the ice there had melted within the last 416,000 years. The walls of the ice fortress had failed much more recently than had been previously imagined possible.
"Once we made the discovery at Camp Century, we thought, 'Hey, what's at the bottom of GISP2?,'" Bierman said.
Though the ice and rock in that core had been studied extensively, "no one's looked at the 3 inches of till to see if it's soil and if it contains plant or insect remains," he said. So he and his colleagues requested a sample from the bottom of the GISP2 core held at the National Science Foundation Ice Core Facility in Lakewood, Colorado.
The study is confirmation that the 2016 "fragile Greenland" hypothesis is right. And it deepens the reasons for concern, showing that the island was warm enough, for long enough, that an entire tundra ecosystem, perhaps with stunted trees, established itself where today ice is two miles deep.
"We now have direct evidence that not only was the ice gone, but that plants and insects were living there," Bierman said. "And that's unassailable. You don't have to rely on calculations or models."
Other collaborators include NASA/Goddard Institute for Space Studies, Lamont Doherty Earth Observatory, University of Washington, Williams College, Purdue University, Geological Survey of Denmark and Greenland and the University of New Hampshire. The work was funded by the National Science Foundation and the Ice Core Facility.
